1. Field of the Invention
The present invention relates to apparatus and method for forming a color image (on a recording medium or for viewing) from an electrical signal and more particularly to improvements in electronic color imaging apparatus and method of the kind using an array of individually addressable light valves.
2. Brief Description of the Prior Art
My U.S. Pat. No. 4,229,095 discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. Thus an array of such light valves comprises a panel of PLZT material with a plurality of interleaved electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing of such light valves in synchronization with the energization of red, green and blue exposing sources and according to the red, green and blue color information for the pixels of that image.
My above-mentioned patent also teaches that, for continuous tone imaging, the electrical energization of the light valves can be varied to provide density variations, i.e., a gray scale. Three exemplary modes for varying such electrical energization are described, viz (1) varying the voltage level that is continuously applied during a nominal exposure period; (2) varying the period of voltage application with a nominal voltage magnitude; and (3) providing a stepped voltage signal during each exposure period and energizing the light valves during the stage of the period when the desired voltage level exists.
In certain applications, e.g. where it is desired to produce high quality continuous tone images such as photographic prints, a detractive artifact has been noted in imaging with light valve arrays that are electrically addressed by the above-described techniques. Specifically, in continuous tone areas a visible density difference is sometimes evident between adjacent pixel regions which have been exposed by light valves that received substantially identical electrical energizations. When exposure is made with relative movement between the imaging media and linear light valve arrays, these density differences are particularly detractive, appearing as streaks or bands.